THE BOT-FLIES. 557 



The DRONE-FLY (Eristahs tenax) belongs to this family. This insect bears a wonderful 

 resemblance to the hive-bee, and has a habit of moving the abdomen in a manner that 

 leads an unaccustomed observer to fancy that it possesses a sting. The larva of this 

 insect is popularly known by the name of Rat-tail Maggot, on account of its peculiar 

 construction. This larva resides in mud, with the head downwards. In order to enable 

 it to breathe, the respiratory tubes are carried into a long and telescopic appendage 

 attached to the tail, the end of which is furnished with a brush of hairs something like 

 that on the tail of the gnat larva. The extremity of this curious organ is always held 

 out of the muddy water, and it is most curious to see the grubs elongate their tails as the 

 depth of water is increased. 



All the vast family of Muscidee, or Flies, are members of this order, and as at least 

 eight hundred British species of this one family are known, it may be imagined that no 

 description of them can be attempted. 



On the right hand of the engraving may be seen a large and bold -looking fly. This 

 belongs to the family of the OZstridse, and is popularly known by the name of BOT-FLY. 

 All these insects are parasitic in or upon animals. The. larva of this Bot-fly resides in the 

 interior of horses, and is conveyed there in a very curious manner. The parent fly deposits 

 her eggs upon the hairs near the shoulders of the horse, where the animal is sure to lick 

 them, in order to rid itself of the unpleasant feeling caused by agglutinated hairs. The 

 eggs are thus conveyed to the stomach, to the coats of which organ the larvse cling, and 

 there remain until they have attained their full growth. They then loosen their hold, 

 are carried, together with the food, through the interior of the animal, fall to the ground, 

 and immediately begin to burrow. They remain underground until they have undergone 

 their metamorphoses, and then emerge in the shape of the perfect insect. They do not 

 seem to inflict any damage upon the animal from whose bodies they have drawn their 

 nourishment, and some veterinary surgeons believe that they are rather beneficial than 

 injurious. 



Another kind of Bot-fly ((Estrus bovis) resides in the cow, but instead of being taken 

 into the stomach, it burrows into the skin, and there forms large tubercles, that are 

 popularly called worbles or wurbles. An aperture is always left on the top of the 

 tubercle, and the larva breathes by means of keeping the two principal spiracles opposite 

 to the orifice. When full grown, they push themselves out of the aperture, fall upon the 

 ground, and there burrow and undergo their transformations. 



The spiracles, to which allusion has been often made, are the apertures through which 

 air is admitted to the system. Insects breathe in a very remarkable manner, the air 

 being conducted through curiously-constructed vessels to every part of the body, even to 

 the extremities of the feet and antennae. It will be seen that the structure of these vessels 

 must be veiy remarkable, on account of the opposite duties they have to perform. As 

 they penetrate the entire insect, it is needful that they should be flexible, in order to 

 permit the creature to move about at will, as if they were stiff-walled the joints would 

 be rendered useless, and the insect would be unable to move a limb. Another character- 

 istic, however, is required. They must be always kept sufficiently open for the free 

 passage of air, and it is not easy to see how these qualities should be united, as a flexible 

 tube will mostly, if abruptly bent, as is continually the case with the air-tubes of the limbs, 

 lose its roundness at the angle, and shut off the communication. An india-rubber gas- 

 tube is a familiar instance of this property of flexible kibes. 



The difficulty is, however, surmounted by a simple and yet most effectual plan. The 

 tubes are double, one within another, and in the interspace a fine but very strong hair- 

 like thread is closely wound in a spiral. It will be seen that, by means of this structure, 

 the tube can be bent in any direction without losing its roundness. The long flexible 

 tubes of Turkish pipes are made in a similar manner, a spiral wire forming the basis, 

 upon which is sewn the leather and silken outer tube one of the many instances where 

 the art of man has been anticipated in the animal creation. A third species ( (Estrus ovis) 

 is parasitic in the sheep, inhabiting the frontal sinus, i.e. the open space between the bcnes 

 on the forehead and between the eyes. 



